1. Field of the Invention
This invention relates to improvements in a valve rocker cover, and more particularly to a valve rocker cover made of a plastic and of the type of being formed at its roof section with a plurality of openings through which spark plugs are installed to a cylinder head on which the valve rocker cover is mounted.
2. Description of Prior Art
Internal combustion engines of the type wherein spark plugs are vertically installed onto a cylinder head have been well known as disclosed, for example, in Japanese Utility Model Publication No. 59-157516. Such engines require a valve rocker cover formed with openings through which the spark plugs are inserted to be installed to the cylinder head. The valve rocker cover has been usually formed of a metal. Each opening is defined inside a cylindrical wall section integral with the main body of the valve rocker cover. The cylindrical wall section is sealingly connected to a metal pipe extending from the cylinder head so that the opening is in communication with the inside of the metal pipe, isolating the inside of the valve rocker cover from the opening. Accordingly, blow-by gases in the valve rocker cover are prevented from leaking out through the opening.
Recently, such a valve rocker cover has been formed of plastic or synthetic resin for the purpose of weight-lightening and cost reduction. With the plastic-made valve rocker cover, the rocker cover side cylindrical wall section is disposed coaxial with and around the cylinder head side metal pipe, in which an annular oil seal is disposed between them to maintain a gas tight seal thereby interrupting communication between the inside of the valve rocker cover and atmospheric air. Such an oil seal member is subjected to vibration of the valve rocker cover itself and vibration transmitted through the metal pipe from the cylinder head, and therefore the oil seal member is required to be securely and rigidly supported to the rocker cover side cylindrical wall section to prevent blow-by gases from leaking out of the valve rocker cover and to prevent the oil seal member from getting out of place. Additionally, the valve rocker cover is made of plastic and fabricated by a molding, so that the cylindrical wall section has a draft and unavoidably has deformation and an imperfect right circular formation in cross-section upon the molding. It will be understood that it is difficult to fabricate the oil seal member in the shape corresponding to the draft, deformation and the like. Accordingly, no tight contact part is formed between the cylindrical wall section and the oil seal member, and nonuniform internal stress is developed upon elastic deformation in use. As a result, insufficient tight contact and securing of the oil seal member relative to the cylindrical wall section will occur, thereby causing danger of blow-by gases leaking and the oil seal member getting out of place.